The following apparatuses are known as apparatuses for forming a visible image on a recording material such as paper based on electric signals output from image information output devices, for example, a computer, word processor and facsimile machine. Namely, an image forming apparatus employing an ink jet method in which ink is used, an image forming apparatus employing a heat transfer method in which ink is fused and transferred, an image forming apparatus using a method of sublimating ink, and an image forming apparatus using an electrophotographic method.
Among these apparatuses, in recent years, the ink jet method which is a non-impact method and performed with a relatively simplified structure including an integrated printer head has been frequently employed to meet demands for an improvement of image quality, an increase in the printing speed and a reduction in cost. However, with the ink jet method, since ink in liquid form is used, recording paper tends to have ink blots which prevent the formation of quality images. Moreover, when different colors are superimposed in color printing, it is difficult to perform good mixed-color development by mixing inks of different colors. Therefore, when high-quality images are desired, the electrophotographic method which performs printing with toner is adopted.
In printing using toner, not only visually excellent strong color images without ink blots are obtained, but also satisfactory mixed colors are obtained because mixing of a plurality of colors is performed in the fixing process of color imaging. Then, a direct printing method has been proposed. This is a combination of a simplified process of the ink jet method and an imaging method using a toner, and performs direct printing by flying the toner.
For example, Japanese Publication for Unexamined Patent Application 191780/1992 (Tokukaihei 4-191780) as a first prior art discloses a structure including a substrate having a plurality of toner passing holes for controlling the passage of toner according to image signals and toner supply means which is provided in the holes only when performing imaging so as to prevent the toner passing holes from being clogged with the toner.
Japanese Publication for Unexamined Patent Application 216963/1992 (Tokukaihei 4-216963) as a second prior art discloses a technique for forming color images on a recording material by a structure including a substrate having a toner passing hole for controlling the passage of toner according to image signals and a plurality of toner tanks which are sequentially moved to the toner passing hole so as to prevent the toner passing hole from being clogged with the toner.
Japanese Publication for Unexamined Patent Application 268591/1992 (Tokukaihei 4-268591) as a third prior art discloses a structure which includes toner tanks for storing toners of different colors disposed in parallel on a transport path of a recording material and a substrate having a toner passing hole in each toner tank, and controls the passage of toner through the toner passing hole according to image signals.
Japanese Publication for Unexamined Patent Application 234233/1994 (Tokukaihei 6-234233) as a fourth prior art discloses a structure which includes toner tanks for storing toners of different colors disposed in parallel on a transport path of a recording material and toner passing holes formed on a common substrate facing the toner tanks so as to correspond to the respective toner tanks, and controls the passage of toner through the toner passing holes according to image signals.
However, with the first and second prior arts, since a control electrode provided in the toner passing hole and the toner tank move with respect to each other, it is difficult to place them in correct positions with accuracy when forming an image. Moreover, since means for performing relative movements of the toner passing hole and the toner tank is required, the structure becomes complicated and the cost is increased.
With the third and fourth prior arts, since the toner tanks for different toner colors are fixed, the above-mentioned problem is solved. However, the first to fourth prior arts including the first and second prior arts do not much consider an improvement of the image quality, for example, the production of quality images by stabilizing an electric potential around the control electrode by reducing the influence of toner, or the production of a vivid color image by forming dots with toner in correct positions.